Organic EL display devices are now attracting attention as components of next generation flat panel displays. Able to perform image display with a wide view angle and a high-speed response and capable of being made thinner and lighter, the organic EL display device, which is a self-luminous type display device utilizing electroluminescence from an organic compound, has become a major focus of research and development in recent years.
The organic EL light emitting element operates on a voltage applied between a first electrode and a second electrode that are disposed opposite to each other or an electric current flowing therebetween. At this time, the electric field tends to concentrate on an edge portion of the electrode that is smaller in radius of curvature and accordingly, undesirable phenomena such as dielectric breakdown and leak current are likely to occur at such an edge portion.
To suppress these phenomena, there is a known technique that is intended to cover the edge portion of the first electrode with an insulating layer. This technique serves to alleviate the electric field concentration that can occur at the edge portion of the electrode. Furthermore, the insulating layer can be produced in such a manner that its thickness in the exposed boundary portion on the first electrode gradually increases from the boundary to form a forward tapered cross-section. This allows an organic light-emitting layer and a second electrode to be deposited smoothly after the formation of the insulating layer and serves to suppress the electric field concentration at the edge portion of the electrode.
A positive type photosensitive resin composition is commonly used as material for the insulation layer. Generally, the effective intensity of radiation into a coating film during exposure gradually decreases from the coating film surface to the bottom portion. Therefore, a positive type photosensitive resin composition can form a forward tapered shape more easily than negative type or non-photosensitive type ones because the exposed portion is dissolved and removed. Among others, positive type photosensitive resin compositions containing polyimide- or polybenzoxazole-based resin have been preferred because the resin is high in heat resistance and little gas generation occurs from cured film, making it possible to produce an organic EL display device with high reliability (see, for example, Patent document 1).
In addition, for the purpose of producing organic electroluminescent devices with increased contrast, attempts have been made to develop an insulating layer with light shielding ability, and a positive type colored photosensitive resin composition containing a light-shielding colorant has been proposed (see, for example, Patent Document 2). To impart light shielding ability necessary for enhancing the contrast, it is necessary to add a considerable amount of a coloring agent to the composition, but the radiation used for exposure is absorbed by the coloring agent and the photoreaction necessary for the pattern formation hardly occurs near the bottom of the film, resulting in a large reduction in sensitivity.
In negative type colored photosensitive resin compositions, which are generally used for the black matrix or the like of a liquid crystal display device, on the other hand, radicals generated by irradiation undergo chain reaction to insolubilize the exposed portions and accordingly, pattern formation can be achieved with relatively high sensitivity compared with positive type ones even when using a composition containing a coloring agent. When using a negative type material, however, the degree of curing reaction tends to be insufficient at the bottom of the film compared to the surface, and the insulating layer is likely to have a backward tapered shape. This induces disconnection of the second electrode formed on the insulating layer, easily leading to defective light emission.
In view of these points, negative type colored photosensitive resin compositions that contain an acrylic resin or a cardo resin and can form a forward tapered shape have been proposed (see, for example, Patent documents 3 and 4).